Zhimin Song

A repetitive X-band relativistic backward-wave oscillator

A repetitive X-band relativistic backward-wave oscillator (BWO) driven by a SINUS-881 accelerator is described. Relativistic electron beams with peak current of 5.4 kA and voltage of 610 kV at a repetition rate of 100 Hz were generated by the SINUS-881 and then guided through the corrugated waveguide by an axial magnetic field of 3.0 T produced by a superconducting magnet. An electron collector was used to collect the electron beams in order to mitigate the effect of secondary emission electrons and to prevent ionization and breakdown near the electron beam dump. This BWO produces a microwave pulse power of 1.1 GW at a 100-Hz repetition rate, a frequency of 9.38 GHz, a pulse duration of 23 ns, and a power transforming efficiency of 33%.

RF phase control in a high-power high-efficiency klystron-like relativistic backward wave oscillator

The klystron-like relativistic backward wave oscillator (RBWO) with a dual-cavity extractor has demonstrated 10 GW output power with 48% efficiency in recent simulations. To realize radio-frequency (RF) phase control for the purpose of coherent power combining, a high-power high-efficiency klystron-like RBWO with an input signal is presented. In the device, an independent input cavity is introduced before the resonant reflector to reduce the leakage power from the input port, and the applied RF signal is injected into the input cavity through a mode convertor and a coaxial waveguide along the magnet, which avoids separating the magnet by the injection waveguide, and suppresses the asymmetric modes excited in the input cavity. For a frequency difference of 15 MHz between the input signal and free-running klystron-like RBWO, an input power of 10 MW is sufficient to control the phase of 10 GW output microwave.

A high-efficiency overmoded klystron-like relativistic backward wave oscillator with low guiding magnetic field

A klystron-like relativistic backward wave oscillator with a ratio of transverse dimension to free-space wavelength being about four is presented. In the beam-wave interaction region, the electron beam interacts with surface wave and volume wave simultaneously. The cathode holder plays an important role in the reflection of backward waves. A guard electrode, an electron collector ring, and a reflection ring are used to optimize the beam-wave interaction. The particle in cell simulation results reveal that microwaves with a power of 2 GW and a frequency of 12.3 GHz are generated with an efficiency of 42% when the diode voltage is 400 kV, the beam current 12 kA, and the magnetic field 0.48 T.

Phase locking of high power relativistic backward wave oscillator using priming effect

The phase-locking approach using the priming effect is developed for high power relativistic backward wave oscillators (RBWO). A plasma switch is conceived to avoid the feedback effect. In experiment, multicavity RBWO of 200 MW with the 73 MHz half power bandwidth is phase-locked under the injection power ratio 0.044 for the frequency separation of 20 MHz. We found that it takes more time to reach stable phase-locking than to achieve saturation of RBWO generation. The external signal of higher power results in the longer time duration of phase locking. Besides phase-locking, the priming effect leads to longer microwave pulse duration.

Investigation of an improved relativistic backward wave oscillator in efficiency and power capacity

Investigation of relativistic backward wave oscillator with high efficiency and power capacity is presented in this paper. To obtain high power and high efficiency, a TM021 mode resonant reflector is used to reduce the pulse shortening and increase power capacity to about 1.7 times. Meanwhile, an extraction cavity at the end of slow wave structure is employed to improve the efficiency from less than 30% to over 40%, through the beam-wave interaction intensification and better energy conversion from modulated electron beam to the electromagnetic field. Consistent with the numerical results, microwave with a power of 3.2 GW, a frequency of 9.75 GHz, and a pulse width of 27 ns was obtained in the high power microwave generation experiment, where the electron beam energy was configured to be ∼910 kV and its current to be ∼8.6 kA. The efficiency of the RBWO exceeds 40% at a voltage range of 870 kV–1000 kV.

A Ka-band TM02 mode relativistic backward wave oscillator with cascaded resonators

By combining the Cerenkov-type generator with the cascaded resonators, this paper proposes a Ka-band relativistic backward wave oscillator operating under the guide magnetic field 1.0 T with high power handling capability and high conversion efficiency. It is found that TM02 can be selected as the operation mode in order to increase the power handling capability and provide sufficient coupling with the electron beam. In slow wave structure (SWS), ripples composed of semicircle on top of the rectangle enhance the wave-beam interaction and decrease the intensity of the electric field on the metallic surface. Taking advantage of the resonator cascades, the output power and the conversion efficiency are promoted greatly. The front cascaded resonators efficiently prevent the power generated in SWS from leaking into the diode region, and quicken the startup of the oscillation due to the premodulation of the beam. However, the post cascade slightly postpones the startup because of the further energy extraction f...

An overmoded relativistic backward wave oscillator with efficient dual-mode operation

A dual-mode operation mechanism in an overmoded relativistic backward wave oscillator is presented. The electron beam interacts with the −1st space harmonic of TM01 mode synchronously in the slow wave structure. Then the backward propagating TM01 mode is converted to the forward propagating TM02 mode. As the phase velocity of the volume harmonic of TM02 mode is about twice that of the surface harmonic of TM01 mode, the TM02 mode also plays an important role in the high-power microwave generation. Particle-in-cell simulation shows that an efficiency of 48% and a significant improvement of the power capacity have been obtained.

Suppressing RF breakdown of powerful backward wave oscillator by field redistribution

An over mode method for suppressing the RF breakdown on metal surface of resonant reflector cavity in powerful backward wave oscillator is investigated. It is found that the electric field is redistributed and electron emission is restrained with an over longitudinal mode cavity. Compared with the general device, a frequency band of about 5 times wider and a power capacity of at least 1.7 times greater are obtained. The results were verified in an X-band high power microwave generation experiment with the output power near 4 gigawatt.

Power combiner with high power capacity and high combination efficiency for two phase-locked relativistic backward wave oscillators

To realize power combination of two phase-locked relativistic backward wave oscillators (RBWOs), a compact power combiner is designed and investigated by 3-D particle-in-cell (PIC) simulation and experiment. The power combiner consists of two TM01-TE11 serpentine mode converters with a common output. When the two incident ports are fed with TM01 modes with a relative phase of 180° and power of 2.5 GW at each port, the conversion efficiency from the incident TM01 modes to the combined TE11 mode is 95.2% at 9.3 GHz, and the maximum electric field in the combiner is 714 kV/cm. The PIC simulation shows that the output power from the common port is 4.2 GW when the power combiner is connected to the two RBWOs with input signals, both producing 2.2 GW microwave, corresponding to a combination efficiency of 95.4%. In the high power microwave test, a method is proposed to obtain the combination efficiency without breaking the vacuum, which is 94.1% when the two phase-locked RBWOs output 1.8 GW and 2.2 GW. The powe...

High-efficiency coaxial relativistic backward wave oscillator

This paper studies the coaxial relativistic backward wave oscillator (CRBWO) through analytical, numerical, and experimental methods. This new type of device is remarked by its high efficiency of more than 35%, which is predicted by the theoretical calculation and the numerical simulation and validated by experiment. The two primary hindrances preventing CRBWO from achieving the expected high efficiency, the poor coaxiality and the power capacity, are discussed in detail and some advanced methods are developed. The theoretical and numerical conclusions agree with the experiment results, which are obtained from the electric probe and the calorimeter simultaneously for each shot of CRBWO. Employing the electron beam pulse of the full width at half maximum 28 ns, a microwave pulse of the width about 20 ns is generated in the experiment; the power is 710 MW and the efficiency is higher than 33%.